INVESTMENT IN MINERA SANTA CRUZ S.A. ("MSC") - SAN JOSE MINE - Changes in Company's Investment in MSC (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Change in the investment in MSC
Investment in MSC, beginning balance	$ 127,814
Income tax recovery	118	$ (1,138)
Distributions received	(2,020)	(4,851)
Investment in MSC, ending balance	123,484		$ 127,814
MSC
Change in the investment in MSC
Investment in MSC, beginning balance	127,814	150,064	150,064
Attributable net income (loss) from MSC	(1,358)	430	(10,065)
Amortization of fair value increments	(1,913)	(2,115)	(9,730)
Income tax recovery	961	$ 1,473	7,930
Distributions received	(2,020)		(10,385)
Investment in MSC, ending balance	$ 123,484		$ 127,814